Amazonian Bacterial Communities after Ecosystem Conversion

by Caroline Vurlumis

The Amazon rainforest is the most extensive tropical forest worldwide containing the largest amount of plant and animal diversity. For years there has been deforestation to create agriculture and cattle pasture which greatly harms biodiversity and causes homogenization in bacterial communities. Rodrigues et al. (2013) tested the impacts of soil microbial biodiversity when land is converted from forest to agriculture. Using transects from forest and pasture, the authors took soil samples of each to test local (alpha) diversity and differentiation (beta) diversity by concentrations of taxonomy and phylogeny. The results showed an increase in alpha diversity and decrease in beta diversity indicating a significant difference in bacterial communities when conversion occurs. There was significant loss in endemic species diversity and an increase of homogenization in the soil which poses a higher risk for net loss of biodiversity in the future. As a result of this study the authors argue that microbial biodiversity loss should be strongly considered when engaging in land conversion due to its important role in tropical ecosystems.

This study took place at Fazenda Nova Vida in Rondonia, Brazil. In a previous study bacterial diversity was not depleted during conversion of the Amazon but these findings were based on a limited sampling depth and local diversity. To prove that animal and plant diversity actually decrease after ecosystem conversion, Rodrigues et al. designed a study without these limitations. They took 10-km transects of both forest and pasture and from these used the spatially nested scheme to take 36 samples of soil from each. Bacterial communities were characterized by barcoded pyrosequencing and then put into taxonomic units. Community diversity and similarity were calculated for both taxonomic and phylogenetic traits. Based on the samples, the authors estimated the local (alpha) diversity, and differentiation (beta) diversity between the soil samples.

The results showed that the forest and pasture soil samples contained significantly different amounts of taxonomic and phylogenetic structures. Converting Amazon forest land to agricultural land resulted in a large decrease of the phyla Acidobacteria, Nitrospirae and Gemmatimonadetes. The phyla Firmicutes, Actinobacteria and Chloroflexi, on the other hand, all increased during conversion. When comparing environmental characteristics, forest soils and their microbacterial communities had lower pH, carbon, and more aluminum, while pasture communities increased in carbon, nitrogen, and magnesium concentration. Overall there were significantly more bacterial communities found in the pasture soils than in the forest soils.

After conversion, bacterial communities were significantly altered between forest and pasture. The decrease in differentiation (beta) diversity indicates increasing similarity in the community (biotic homogenization). The loss of endemic taxa and increase in similarity between phylogenetic bacteria suggests a higher risk of biodiversity decline during conversion. A higher homogenization increases risk since there is less resilience to disturbance which could lead to a net loss of diversity. As more soil is sampled, the forest surpasses pasture samples in taxonomic richness. To fully argue this, more sampling needs to be done to document diversity loss. In this study there was a minimal difference detected but it nevertheless represents a huge loss of genetic variation after ecosystem conversion. There was a significant reduction in forest endemic species while homogenization of taxa increased. A change of microbial diversity through conversation should be of concern since microbes play an important function in the ecosystem. Further studies need to be conducted to find out the long-term impacts of biodiversity loss.

Rodrigues, J., Pellizari, V., Mueller, R., Baek, Kyunghwa., Jesus, Ederson., Paula, Fabiana., Mirza, B., Hamaoui, George., Tsai, Siu., Feigl, B., Tiedje, James., Bohannan, B,. Nüsslein, K., 2013. Conversion of the Amazon rainforest to agriculture results in biotic homogenization of soil bacterial communities. Proceedings of the National Academy of Sciences 110, 998–993. http://www.pnas.org/content/110/3/988.full

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